In a production technology for high quality sheets, removal of non-metallic foreign matter or impurity during the molten steel state is critical for determining fraction defective of the products. It is recent trend in molten steel purification technologies,
(1) to increase size of an intermediate container, i.e. tundish, between a ladle and a mold in a continuous casting to prolong a period to maintain the molten steel in the tundish with the expected floating up of the foreign matter; PA0 (2) to provide gates in multi-stages in the tundish for controlling flowing route of the molten steel to prolong period to maintain the molten steel in the tundish; and PA0 (3) in the mold, to prevent mold powder generated by molten steel flow from a discharge opening of a nozzle from penetrating by modifying configuration of an immersion nozzle to control flow of the molten steel within the mold. PA0 (1) When the molten steel is horizontally rotated, the outer circumferential portion of the molten steel proturburates in parabolic fashion, the height of which is proportional to square of the radius and rotation speed Therefore, increasing of the radius results in substantial increase of the height of the facility. In addition, in order to drive all of the molten steel for horizontal rotation, substantially large electromagnetic coil is required which increases cost for the facility making it impractical. PA0 (2) Reduction of rotational radius may be desirable in view of the requirements for the facility. However, reduction of the capacity Of the tundish may make it impossible to accomplish the buffer function for realizing ladle replacing. PA0 (3) Due to penetration of air into the molten metal resulting from swirl flow, air oxidation of the upper surface of the molten metal or melting of refractory will be simultaneously progressed to abruptly increase the non-metallic foreign matter generated in the container and to flow out the large size non-metallic foreign matter. As a solution to this, it becomes necessary to use expensive refractory having high wear resistance in the overall area of the container and to seal the overall container with gas or so forth, causing increased cost. PA0 (4) If the molten steel removed from the foreign matter by horizontal swirl flow is simply discharged from the portion of the bottom of a rotary bath in the vicinity of the swirl center of the molten steel, the foreign matter separation effect can be degraded when the molten steel level in the tundish is lowered. PA0 (5) Particularly, in the case that the molten steel is directly poured to the mold from the bottom, namely the bottom surface of the refractory of the rotary bath, it is difficult to obtain a high foreign matter separation effect in the overall range of pouring.
However, with these methods, satisfactory improvement of the quality cannot be obtained. Particularly, the quality at the non-steady pouring, so-called as ladle exchange, is a level creating a problem. Therefore, methods are disclosed in Japanese Unexamined Patent Publications (Kokai) Nos. 58-22317, 55-107743, 01-312024 and 02-217430, to generate a horizontal swirl flow of the molten metal to float up the foreign matter. This technology provides centrifugal force by horizontal rotation to the molten metal and the non-metallic foreign matter so as to concentrate the non-metallic foreign matter toward the swirl center due to difference of specific weights to separate by promoting collision, absorption and aggregation. This technology can achieve an improvement in the foreign matter separation effect in comparison with the methods simply prolonging dwell period or controlling molten steel flow path in the tundish. In other words, when an equal separation capacity is required, the last-mentioned method may provide an advantage in significant reduction of the size of the tundish.
On the other hand, the technology disclosed in Japanese Unexamined Patent Publication No. 58-22317 simply provides a rotational force generating apparatus outside of the tundish. On the other hand, the technologies disclosed in Japanese Unexamined Patent Publications Nos. 55-107743, 01-312024 or 02-217430, simply provide energization coils in the outer circumferences of the tundishes, and do not disclose concrete facility construction. Accordingly, if such technologies are applied, a problem is encountered in restriction for attaching and detaching power source cables, cooling water paths upon moving the tundish for the repairing or so forth, the magnitude of movement of which can be substantial, because the rotational force generating apparatus or the energization coil have to be moved therewith.
Especially, in case of the apparatus for electromagnetically providing rotational force (energization coil), connection of the cable is a labor intensive operation, and the operation is very difficult. On the other hand, they may provide an advantage to permit preliminary adjustment of positional relationship between the tundish and the coil. However, the above-mentioned problem is much more critical.
On the other hand, the above-mentioned method for purifying the molten steel employing horizontal swirl flow as disclosed in Japanese Unexamined Patent Publications 58-22317 or 55-107743, the following problems can be encountered.
On the other hand, as set forth above, in the continuous casting of the molten metal, there have been proposed means for concentrating the non-metallic foreign matter toward the rotation center for separation by rotating the molten metal in the horizontal direction and by utilizing the difference of the centrifugal forces resulting from the difference of densities between the molten metal and slag (see Japanese Unexamined Patent Publication No. 55-107743), or means for separating the non-metallic foreign matter by natural floating up after horizontal rotation (see Japanese Unexamined Patent Publication No. 01-312024).
However, in either case, a molten metal circulating bath 54a of a tundish 54 is positioned in the vicinity of a ladle nozzle 53 so that the ladle nozzle 53 is submerged within the rotating molten metal, as shown in FIGS. 34 and 35. Therefore, the ladle nozzle 53 may be subject to melting or breakage due to the force resulting from flow velocity of the molten metal 51. In FIG. 34, 58 denotes a tundish nozzle, 59 denotes a mold, and 60 denotes a cast block.
In addition, the method of purifying the molten steel employing the horizontal swirl flow as set forth above, further holds the following problems.
This is the same either in the case that the pouring is performed from the rotary bath directly to the mold or in the case that the pouring is performed from the rotary bath to the mold via a floatation bath (distribution path).
On the other hand, a carbon steel is typically used for the tundish, and, in particular, an austenitic stainless steel is used for suppressing attenuation of magnetic field when a static magnetic field is applied (see Japanese Unexamined Patent Publication Nos. 1-279706, 2-217430 and 1-312024).
When a shifting field is applied to the tundish, and if the material of the container member of the tundish is the carbon steel, the magnetic field is attenuated so that the magnetic field cannot be effectively applied to the molten steel within the tundish.
Also, when the container member of the tundish is stainless steel, although attenuation of the magnetic field will not be caused, an eddy current may be generated within the tundish container member in the shifting field. Therefore, a force to move the container is generated to cause vibration of the overall container.
On the other hand, as set forth above, as a method for preventing lowering of the temperature of the molten steel in the tundish and separating the foreign matter by floating up at the center of the tundish with the difference of the centrifugal force resulting from rotational force exerted on the molten steel, apparatus disclosed in Japanese Unexamined Patent Publication No. 01-245019 and illustrated in FIGS. 45 and 46 are proposed by the owner of the present invention. The feature of the apparatus illustrated in FIGS. 45 and 46 resides in a solenoid coil 92 provided around a tundish 91 for heating, and a shifting field generating coil 93 providing stirring.
This apparatus will not create any problem when heating and rotating stirring independently, but will create problems when both are operated simultaneously.
In FIGS. 45 and 46, flow patterns of the molten steel generated in a molten steel 94 when the heating solenoid coil 92 and the stirring shifting field generating coil 93 are operated simultaneously.
The flow pattern of the molten steel 94 generated by the heating solenoid coil 92 is similar to the case of a crucible PG,9 induction furnace as illustrated in "Industrial Electric Heating", published by Foundation of Energy Saving Center, pp 110, FIG. 4.23, in which reversing flow in vertical direction is formed about the solenoid coil 92.
On the other hand, the flow pattern of the molten steel generated by the shifting field generating coil 93 for rotating stirring is swirl flow 96 in the horizontal direction.
Accordingly when the heating coil 92 and the stirring coil 93 are operated simultaneously, the swirl flow 96 in the horizontal direction for separating the foreign matter is disturbed by the vertical reversing flow generated by the operation of the heating coil 92. As a result, the swirl flow 95 in the horizontal direction is weakened to lower the performance for separating the foreign matter.
On the other hand, as set forth above, a technology for separating the foreign matter in the tundish of the continuous casting facility by floating up, which can be an important point in determining the quality of the product, has been disclosed in Japanese Unexamined Patent Publication No. 1-312024. Namely, it can employ a method, in which, as shown in FIGS. 49 and 50, in a rectangular shape tundish 110, a semi-cylindrical coil device 101 for generating a shifting field is provided on the outer periphery of a swirl flow bath 110a as a bath for pouring the molten steel from a ladle 105 for stirring molten steel 106 in the above-mentioned bath 110a to float up the foreign matter having small specific weight with the centrifugal force. 102 denotes a molten steel path, 103 denotes an iron skin, 104 denotes a refractory, 107 denotes a submerged nozzle of a ladle, 108 denotes a submerged nozzle of the tundish, 109 denotes an arrow indicating rotating direction of the molten steel, and 110b is a distributing bath.
With this arrangement, when swirl flow 109 is generated in the molten steel 106, the swirling molten steel surface 106a becomes a concaved surface depending upon the rotation speed as illustrated in FIG. 48. 106b denotes a static molten steel surface. The depth Z(m) of the concaved surface shown in FIG. 48 can be expressed by the following equation, assuming the rotation speed of the molten steel is N (r.p.m.), a rotation radius is r(m) and the gravity weight is g: ##EQU1##
As a problem to be created by causing swirling surface 106a (concaved surface) of the molten steel, defects in that the excessive length of the submerged nozzle 107 for pouring the molten steel 106 from the ladle 105 without causing oxidation is required which raises the cost for the nozzle and in that possibility of causing breakage due to thermal impact and so forth is increased.
In addition, by formation of the concaved surface, the area of the molten steel surface 106a is increased to cause a problem in promoting oxidation of the molten steel surface 106a.
On the other hand, in the example of FIGS. 49 and 50, since the configuration of the tundish 110 is specified, sufficient rotational force can be obtained with the shifting field generated by the semi-cylindrical coil device 101. However, the configuration of the tundish is not limited to the configuration illustrated in FIGS. 49 and 50, and can be of the configurations as illustrated in FIGS. 53 and 54.
When the coil device 101 is provided for applying the rotational force for the molten steel in the swirl flow bath 110a of the tundish 110 in the configuration as illustrated in FIGS. 53 and 54, since the outer periphery of the swirl flow bath 110a is divided into two sections by the distribution bath 110b at both sides in either case, each coil device 101a, 101b, 101c and 101d can not cover the 180.degree. of angular range of the swirl flow bath 110a.
Here, discussion will be given for the principle of application of the rotational force for the molten steel with the shifting field in terms of the linear type shifting field generating coil device shown in FIG. 55. The coil generally has two poles so that a magnetic flux 113 flows from an electrode 111 to an electrode 112. 114 denotes an iron core and 115 denotes a winding coil. An eddy current generated by the shifting field is caused in the direction perpendicular to the paper surface. Then, on the molten steel 106, a force 118 in the horizontal direction, which is directed in the shifting direction of the shifting field and a depression force 119 in a direction perpendicular to the shifting direction are exerted. The component of magnetic flux density for generating the force 118 in the horizontal direction is the component 120 in the perpendicular direction to the molten steel 106.
Accordingly, in order to provide effective rotational force for the molten steel 106 by the shifting field, it is necessary to make the magnetic flux density component 120 in the perpendicular direction to the molten steel 106. In order to increase this component, it is generally required to enlarge a pole pitch 121 (in case of the coil having two poles, one half of a coil length 122) of the shifting filed generating coil device and thus to increase the coil length 122.
In case of the coil arrangement as illustrated in FIG. 55, since the length 122 of the coil device is shorter than the arrangement illustrated in FIGS. 49 and 50 as set forth above, the magnetic flux density component 120 in the perpendicular direction to the molten steel 106 becomes smaller. Therefore, the rotational force to be exerted on the molten steel 106 becomes smaller to make it difficult to separate the foreign matter from the molten steel 106.
On the other hand, in the above-mentioned Japanese Unexamined Patent Publications Nos. 01-312024 and 02-217430, the outer shell of the coil device is formed of a metal having small magnetic loss, such as an austenitic stainless steel or so forth, which outer shell is arranged in direct opposition to the molten metal container. The coil device has a coil body 151 within a casing 152 as shown in FIG. 57, for example. The casing 152 is formed of a metal.
Besides, in the method employing a conductive body, such as the metal, for forming the casing of the coil device, the eddy current can be generated within the casing member to cause heat generation to create problems of lowering of strength of the casing or burning out of the coil body within the casing.
On the other hand, when the metal casing of the above-mentioned coil device is exposed, the heat radiated from the tundish of the molten metal is directly received by the metal casing of the coil device to cause failure of the coil device. In addition, when the molten metal overflows from the tundish for the molter, metal, it may cause a problem of melting off of the coil device.
On the other hand, when the coil device is arranged in the close proximity of the circumference of the molten metal container as set forth above, problems of lowering of the casing and lowering of the performance of the coil device due to direct transmission of the radiation heat from the molten metal container, and of rising of the temperature of the molten metal container member for causing lowering of the strength, can be encountered.
Furthermore, as set forth above, as a known method for avoiding penetration of the non-metallic foreign matter into the metal during casting of the molten metal, a method applying a rotational force with a magnetic force for separating and removing non-metallic foreign matter in the tundish and so forth, in order to prevent the non-metallic foreign matter from being entangled in the molten metal flow shorting to a discharge outlet and having high flow velocity (see Japanese Unexamined Patent Publication No. 58-22317).
On the other hand, at the inlet for the container, in view of avoiding striking in of the oxide covering the molten metal surface into the molten metal, a pouring method employing a nozzle submerging the tip end thereof into the molten metal as shown in FIG. 62 is generally employed. In FIG. 62, 181 denotes a molten metal, 182 denotes a ladle, 183a denotes a long nozzle, 184 denotes a tundish, 186 denotes a submerged nozzle, 188 denotes an upper lid, and 193 denotes a gate.
However, the swirling molten metal forms the concave at the swirl center, when the nozzle is submerged to the swirl center, if the length of the nozzle 183a is excessive in the extent to reach the bottom of the container, it causes increasing of the cost for the refractory and difficulty in maintaining strength. When the nozzle is submerged at the position offset from the swirl center for avoiding the foregoing problem, a possibility of damaging of the nozzle due to rotational force of the molten metal cannot be ignored.
Namely, as exemplarily illustrated in FIG. 63, in the pouring of the molten metal without using the submerged nozzle, a seal pipe 194 used for the purpose of protecting the poured molten metal stream from air oxidation generally, is provided with a diameter four to five or more times greater than the ladle nozzle in view of reduction of the cross-sectional area due to metal splashing. Therefore, upon replacing of the ladle, an opening to communicate with the atmospheric air becomes large to permit air to be contained within the container. The increased oxygen and nitrogen concentration in the container may encounter a problem of degradation of the quality of the cast block at the non-steady state portion, Also, even at the steady state portion, since there are a lot of portions requiring a seal between the ladle and the seal pipe, the seal can becomes incomplete even if the inert gas introduction pipe 189 is provided to similarly cause the problem of penetration of the air.
In addition, an apparatus disclosed in Japanese Unexamined Patent Publication No. 1-278706 is illustrated in FIG. 67, in which the centrifugal force is exerted on the molten steel by applying the horizontal rotational force to the molten steel in the tundish for floating up and separating the foreign matter in the molten steel to the tundish center with the concentric force due to difference of the specific weight. For the molten metal 207 poured through the nozzle 202 from the ladle 201 to the tundish 203 is generated the horizontal swirl flow 206 by the shifting field generating coil 209 to float up and separate the foreign matter and to extract a purified steel via a tundish nozzle at a position offset from the swirl center of the molten steel 207.
In the conventional method illustrated in FIG. 67, the molten steel 207 in the tundish 203 can be provided with a lid thereon for preventing the air from penetrating as much as possible so as to avoid re-oxidation due to contacting with the air and for preventing splashing upon pouring.
In the construction of the conventional apparatus as illustrated in FIG. 67, it is not only difficult to determine the swirling state of the molten steel 207 in the tundish 203 but also is impossible to control the floating state of the foreign matter by providing proper rotational force at respective process state in a sequence of operation pattern (e.g. initial state of casting, steady state period, ladle replacing state) in the continuous casting facility.
It is a primary object of the present invention to solve the problems in the prior art set forth above and to provide a tundish moving apparatus for a continuous casting of a steel in which has the tundish and the associated facilities allow replacing and repairing of the tundish without being constrained by a power source cable for an energization coil for rotating the molten steel in the tundish, or the cooling water, and the associated facilities thereof.
Also, it is another object of the present invention to provide a tundish moving apparatus for a continuous casting of a steel with a construction, in which a coil is preliminarily installed in a moving table (normally called a tundish car) for moving the tundish, and to receive a detachable tundish in opposition to the coil, and the positioning of the molten steel swirl flow portion in the tundish and the coil opposing to the side wall of the former.
A further object of the invention is to solve the problems set forth above and to provide an apparatus for removing non-metallic foreign matter in a molten metal for effectively and economically realizing separation and removal of the non-metallic foreign matter in the molten metal.
A still further object of the invention is to solve the foregoing problems and to provide a tundish for continuous casting for efficiently separating a slag in the molten metal of from small size to large size.
A yet further object of the invention is to solve the foregoing problems and to provide an apparatus for removing non-metallic foreign matter in a molten metal for effectively realizing separation and removal of the foreign matter in the molten steel either at replacing of a ladle or at a steady state.
A still further object of the invention is to solve the above-mentioned problems and to provide a vibration suppressive tundish for separating and removing non-metallic foreign matter in a molten metal.
A yet further object of the invention is to provide a non-metallic foreign matter removing apparatus for a molten metal which prevents vertical reversing flow from being generated even when a heating coil is actuated and thus certainly maintain a function for separating the foreign matter.
A yet further object of the invention to solve the foregoing problems and to provide a tundish which includes a shifting field generating coil device which can avoid oxidation of a molten steel and certainly maintain a foreign matter separating function.
A yet further object of the invention is to provide a tundish which has a shifting field generating coil device which enhances rotational stirring of a molten steel in the tundish for improving a separation effect of foreign matter in the molten steel.
A still further object of the invention is solve the above-mentioned problem and to provide a shifting field generating electromagnetic coil device with enhanced heat insulation or refractoriness.
A yet further object of the invention is to solve the foregoing problems and to provide a shifting field generating coil device which can avoid lowering of performance or burning of the coil.
A yet further object of the invention is to solve the problem and to provide a non-metallic foreign matter removing apparatus for a molten metal which has a device for promoting heat radiation.
Another object of the invention is to overcome the problems set forth above and to provide a casting method, in which can restrict non-metallic foreign matter to be introduced into a tundish from a ladle and stably perform casting by employing means for actively promoting separation and removal of the non-metallic foreign matter in the tundish, and whereby obtain high quality cast block.
A further object of the invention is to solve the foregoing problems and to provide a processing method of a molten steel in a tundish which can provide proper rotational force at respective operation stage in a molten steel processing in the tundish.